This invention relates to optical stabilization and in particular to a method for optically stabilizing images during the tracking and designation of targets in a laser guided designator system.
Laser Tracking and Designation Processes
A typical laser designator rangefinder system 10 is illustrated in FIG. 1 and consists of three modules; a sighting system or sensor module 12, a laser designation module 14 and a tripod angulation head 16. An operator 18 is typically a forward observer who must carry the laser designator rangefinder system 10 to a remote location, set it up, and then acquire a target of interest which could be moving. The operator 18 must track the target and determine its range in order to obtain a grid coordinate, which is then transmitted to a command station. A laser guided bomb or missile (not shown) is then launched to the grid coordinate provided by the operator 18. The operator 18 will continue to track the target while simultaneously designating it with a laser beam for terminal guidance of the laser guided missile. The operator 18 designates the target by keeping the laser beam fixed on some point on the target. A sensor on the laser guided missile detects the point on the target designated by the operator 18 and guides its trajectory towards that point.
Obviously, to perform these tasks, the laser designator rangefinder system 10 must be maintained in a very stable position, substantially free of extraneous motion relative to the target. This is due to the fact that motion of the laser beam off of the target will cause the laser guided missile to miss the target. Generally it is advisable and typically even necessary to permit low frequency or slow motion of the laser designator rangefinder system 10 with respect to the target in order to allow the operator 18 to follow the system with the target during tracking of a moving target. However, high frequency motion of the laser designator rangefinder system 10 with respect to the target or jitter must be substantially eliminated in order ensure accuracy when tracking and designating a moving target.
During an initial search phase, the operator 18 must search the scene or field of view for a suitable target of interest. During this phase, jitter comprised of relatively high frequencies is undesirably imparted to the laser designator rangefinder system 10 by the operator 18 in moving the system 10 through the field of view.
Therefore, it would be advantageous if jitter during the initial search phase could be reduced beyond that provided by the prior art while reducing the overall weight of the system, thereby enhancing the Modulation Transfer Function (MTF) or optical resolution of the system and range in which potential targets of interest can be recognized using a practical solution.
During a range finding phase, the operator 18 generally selects a button or trigger, which fires the range finder in the sensor module 12. This motion can result in missing the target and receiving false ranges. The target range is important in computing grid coordinates, which will guide the trajectory of the laser guided missile.
Therefore, it would be advantageous if a tracking mode could be engaged, prior to firing a laser used in the range finding phase, which would prevent the optical axis of the laser designator rangefinder system 10 from shifting when the button or trigger used to initiate rangefinding is depressed.
After the target of interest has been obtained during a target tracking phase, the operator 18 must keep the laser beam from the laser designation module 14 on the target while the target is moving until the impact of the laser guided missile during the designation phase. Therefore, it would be advantageous if a track mode could automatically maintain the laser designation module 14 locked onto the target without operator intervention.
In many instances it becomes impractical to utilize the laser designator rangefinder system 10 with the tripod angulation head 16. Such is the case where, for instance, the mobility of the operator 18 is at a premium or the terrain does not permit the use of the tripod. Therefore, it would be advantageous if an alternate stabilization technique was designed to operate independently of the tripod angulation head 16.
Prior Art Methods for Damping Motion in Laser Designator Rangefinder System
One of the methods utilized in the prior art to reduce high frequency motion of the laser designator rangefinder system 10 with respect to the target involves the use of a mechanically viscous damped head coupled to a very sturdy tripod. Such a solution is both heavy (approximately sixteen pounds) and expensive whereas the embodiments of the present invention would add considerably less than one pound.
Therefore, it would be advantageous if motion of a predetermined frequency with respect to the target could be substantially eliminated by using a method that results in a system which is substantially less expensive to manufacture than viscously damped systems. In addition, since the laser designator rangefinder system 10 must be carried by the operator 18 over substantial distances, it would also be advantageous if the weight of such a system could be reduced as well.
In order to reduce further the amount of weight carried by any one operator 18, an ideal system would comprise two separate modules; the sighting module 12 and the designation module 14, which could be carried independently by two operators and then assembled in the field. A problem arises, however, involving the method used in correlating the boresight of the two separate modules once assembled. Relatively precise tolerances must be maintained in order to guarantee positioning of the two separate modules with respect to each other so that the difference in boresight between them can be regarded as substantially constant and factored out during calculations. A substantial mounting fixture must be used in order to maintain such precise tolerances in the positioning of the two modules with respect to each other once assembled, and this too becomes heavy and costly.
Therefore, it would be advantageous if the boresights or optical axes of the sensor module 12 and the laser designator module 14 could be automatically aligned upon completion of assembly in the field. Such an automatic boresighting feature would compensate for greater tolerances in the assembly hardware that mates the sensor module 12 to the laser designator module 14, further reducing system weight and cost over the prior art.
Another solution found in the prior art involves stabilization through electronic means, which is typically used in camcorders. However, such a method cannot be used to stabilize multiple sensors, and requires that a portion of the field of view be sacrificed. Yet another prior art solution involves the use of gyroscopes to stabilize the sensor module or sighting system. For instance, steady cam television camera systems are gyroscopically stabilized to reduce jitter caused by a cameraman operating the system. However, systems that use gyroscopes are typically quite large and heavy, making them impractical for deployment in the field by one or two operators. In addition, the cost of such a system would generally be considered prohibitive for man-portable systems which are typically purchased in large quantities.
Such a concept incorporating the advantages discussed above could be utilized in applications where viscously damped tripods are currently used and where electronic stabilization is not practical. For instance, such a system could provide major benefits if utilized in multispectral multi-sensor systems, including high performance manually controlled tracking systems such as the Long Range Advanced Scout Surveillance System (LRAS3) and other scout systems as well as photographic and cinematographic equipment.
Alternative Guidance Methods without Laser Designation
Laser guided missiles of the prior art typically involve the use of laser designation of the target by a soldier situated in the field. The act of maintaining a beam of light on the target in order to guide the missile to the target places the soldier in an extremely vulnerable position. By actively emitting energy, which is detectable, the soldier becomes a target. In addition, since the soldier is already burdened with heavy laser designation equipment, the amount of additional weaponry he can carry is limited.
Therefore, it would be advantageous if a GPS/INS (Global Positioning/Inertial System) based system would alleviate the need for personnel to carry laser designation equipment in the field and continuously maintain the laser on the target in order to guide the missile.